Apical constriction is a cell shape change that associates with cell ingression, bending of epithelial sheet, and formation of tubular structures. It is found in many morphogenetic processes, such as gastrulation, neural tube closure, and sensory organ formation. Failure in apical constriction can cause human congenital diseases, such as neural tube defects. Despite the importance of apical constriction in multiple developmental contexts, molecular regulators of apical constriction are not understood completely. Rho signaling has been implicated previously in apical constriction during vertebrate neural tube closure and sensory placode invagination. However, general activation of Rho throughout a cell does not lead to apical constriction, underscoring that polarized stimulation of Rho within particular subcellular compartment is crucial. Spatial regulation of Rho activities is normally achieved by Rho regulators GEFs and GAPs. Over 20 members each of RhoGEFs and RhoGAPs perform diverse cellular functions. The identity of Rho regulators in apical constriction in vertebrates is not well defined, and the mechanisms via which Rho regulators act to control cell shape changes are not described in detail. This knowledge gap, combined with the importance of apical constriction in embryogenesis, demands further investigation about molecular machinery controlling apical constriction. In our current study, we identified plekhg5 as a RhoGEF expressed in the bottle cells of the blastopore lip during Xenopus gastrulation and had a function in regulating apical constriction of the bottle cells. Plekhg5 protein is apically localized and stimulates apical actomyosin assembly to induce ectopic blastopore lip in a Rho-dependent fashion when ectopically expressed. Knockdown of plekhg5 blocks apical constriction of bottle cells at the blastopore lip and prevents activin from inducing blastopore lip in the ectoderm. Plekhg5 is thus an endogenous RhoGEF in bottle cells that participates in regulation of apical constriction during gastrulation. The activity of plekhg5 provides us an excellent opportunity to address some of the key issues regarding apical constriction in any tissue contexts, namely how Rho regulators are recruited to particular subcellular compartment(s) to exert their function (aim 1); how they modulate dynamic actomyosin organization to coordinate reduction of apical cell surface and adhesion complex remodeling (aim 2); and how different downstream effectors are involved in regulating distinct aspects of actomyosin dynamics and cell shape changes (aim 3). Completion of the proposed studies will offer us deeper insight into molecular control of apical constriction and provide us a platform to investigate and compare molecular mechanisms governing apical constriction in diverse tissue contexts. The results may also contribute to our understanding of human diseases caused by abnormal epithelial morphogenesis due to defects in apical constriction.